Role of δ-phase on Mechanical Behaviors of Additive Manufactured Inconel 718: Detailed Microstructure Analysis and Crystal Plasticity Modelling

In this work, a multi-scale approach was applied to reveal the evolution mechanisms of the δ (Ni₃Nb) phase and the resultant mechanical responses in wire-arc additive manufactured (WAAM) Inconel (IN) 718. Several heat treatment strategies, consist of homogenization, intermediate heat treatment and aging process, were firstly conducted to produce δ phases with different characteristics. The early evolution of the δ phase was then analyzed in detail via transmission electron microscopy. The nanoindentation and tensile tests were also used to verify the strengthen response of δ phase. Moreover, a crystal plasticity finite element (CPFE) model, taking into account the different configurations of δ phase, was developed to study and predict the micro deformation behavior. Results revealed that when the intermediate heat treatment time exceeded 1 h, the formation of the main strengthening phase γ" was constrained by the δ phase, accompanied by the coarsening of γ". The decrease of γ" phase and the destruction of coherent γ"/γ interface are the main causes of strength loss in the WAAMed IN718. The intergranular dislocation transmission was hindered by the δ phase, thereby increasing the grain boundary strength. However, the needle-like δ phase within the grain may cause premature fracture due to strain localization, thereby reducing the material properties. The evolution mechanisms of the δ phase obtained in this study provides theoretical guidance for the optimization of the microstructures and mechanical properties of WAAM Inconel 718.